How can I make an object a light source in blender 2.8 Cycles?
It is exactly the same process as it was in previous versions. You have to create a new material for your plane, enable Use Nodes for it in the Properties Editor, Material tab and connect an Emission shader to Material Output node in the node tree of the material in the Shader Editor. You might also want to disable light on the back side by mixing the Emission shader with nothing by the Geometry node's Backfacing output as factor:



If it is a simple plane that emits light that you are after it is more logical to use an Area Light. Area lights render faster than geometry lights.